Regarding the positional accuracy of a topographic map provided for map information, it is said that the position may have error of a dozen meters due to human error, measurement error, or the like. Also, it is known that a road elevation is greatly influenced if the position of the tunnel entrance (i.e. entrance and exit) is off the point because, in general, a terrain elevation widely changes around the tunnel entrance. In other words, when the position of the tunnel entrance is recognized erroneously at a position slightly displaced toward the tunnel, the elevation at the slope of the mountain located above the tunnel is erroneously computed as the road elevation of the entrance.
When the above erroneous elevation is used, for example, the road gradient within the tunnel, which is computed by using the elevation, may generate error.
Thus, in order to highly accurately estimate the road elevation, a vehicular road elevation estimating device is proposed (Patent Document 1). The vehicular road elevation estimating device computes a probability of a surface altitude value that is computed from surface altitude data. Then, the vehicular road elevation estimating device executes a filtering process for weighing the data with higher probability to estimate a road elevation as a value closer to an actual road elevation.
Please refer Patent Documents 2 to 4 that disclose conventional techniques related to the present invention.